"Rotting in the field" implies a short time horizon, as if the illegals were all to be made to vanish, in their millions, all within a few months. If it takes any longer than that, such as years, to remove any large proportion of illegals, nothing will rot for lack of pickers; unless they're talking only about orchards. They said fields, though. If groves were not cut down, in the event that pickers' pay were too high to compete with imports' prices, then you would get some rotten fruits and nuts, but not as bad as the quality of the arguments predicting a phony crisis of field crops.

If there is a danger of the "crop rotting in the field," the farmer can change his cropping system. No farmer likes to do this, but good farmland can typically grow a variety of things. If illegal labor becomes scarce, the cost of labor goes up, and the incentive for mechanization increases. In fact, the history of agriculture is in large measure the history of mechanization of harvest. Even the hay baler and the cotton picker are relatively recent inventions. If a machine can assemble a computer, maybe it can learn to pick a tomato!